The present invention relates generally to implantable and external devices for monitoring physiologic parameters associated with respiration. In particular, the present invention relates to monitoring periodic breathing, such as central sleep apnea, as an indicator or biomarker of congestive heart failure.
Sleep apnea is generally defined as a cessation of breathing during sleep, and can typically be categorized as one of two basic types: central sleep apnea (CSA) or obstructive sleep apnea (OSA). OSA is the more common type of sleep apnea, and is generally characterized by repetitive pauses in breathing during sleep due to the obstruction and/or collapse of the upper airway. CSA is an apnea that is neurally mediated. The apnea is caused by neural signaling (or lack thereof) causing cessation of substantially all respiratory effort during sleep, usually accompanied with decreases in blood oxygen saturation (SaO2).
Unlike OSA, CSA does not necessarily involve blockage of an airway. Instead, CSA involves failure of the brain to send appropriate signals to initiate action of the muscles required for respiration. CSA occurs during sleep when an acute increase in ventilation results in a decrease in the level of carbon dioxide in a patient's bloodstream (i.e., the PaCO2). When the PaCO2 falls below a threshold level required to stimulate breathing, the “central” (as in Central Nervous System) drive to respiratory muscles and airflow ceases, initiating central apnea. This apnea persists until the patient's PaCO2 level rises above the threshold required to stimulate ventilation, upon which the cycle of hyperpnea followed by apnea may repeat. This is referred to as “periodic breathing”.
Patients with congestive heart failure (CHF) frequently suffer from CSA as well as similar periodic breathing disorders such as Cheyne-Stokes breathing while asleep or awake. The presence of CSA is a reflection of a compromised cardiac function, with CSA being recognized as a consequence of CHF. See Floras et al., Circulation, 107(2003): 1822-1826. As such, the presence, or worsening in the severity, of periodic breathing may alert a healthcare provider to the necessity of intensifying a patient's CHF therapy.